1. Field of the Invention
The present invention relates to a light pen for use with a cathode ray tube (CRT) forming part of a video display terminal. Specifically, the present invention relates to a compensated light pen that will work without adjustment with substantially all presently used CRTs used in video display terminals.
2. Description of the Prior Art
Light pens in general have a configuration similar to that of a writing pen. However, the light pen uses the light emitting from the cathode ray tube as the communication media between a computer system which incorporates the video display and the operator of such computer system.
The light pen itself does not write with light on the CRT screen of the video display but rather the light pen detects light pulses from the CRT to thereby produce electrical pulses in response to the light pulses. The CRT typically presents information to the viewer by directing an internal beam of electrons to a phosphorescent coating on the rear surface of a glass viewing screen. As the beam of electrons strikes the phosphorescent surface at any particular point, light energy is created on the screen at that point. The electron beam is typically scanned across the entire viewing surface as a series of horizontal lines designated as a raster, so that ultimately the entire internal surface of the CRT screen receives electron beam energy. The beam and its resultant spot of light is continually moving so that at any particular point the light energy is produced for only a very brief period of time.
Ideally, the light pen, when positioned at the desired spot adjacent the CRT screen, would produce a single pulse when the point of light was directly in front of the pen. Also ideally, this single pulse would be independent of the phosphor type for the screen and the light intensity at the particular point. However, the light pen systems currently in use have a number of specific limitations since the light pen has a field of view that encompasses several horizontal lines of the electronic beam raster. Therefore, the output from the light pen is not a single pulse but is typically a series of pulses. Also the apparent field of view for the light pen varies with the light intensity from the CRT. Further, the actual field of the view of the light pen depends on the distance from the pen to the actual phospher layer for the CRT. In addition, the edges of the field of view for the light pen are not well defined so that the pulses tend to fall off to either side of the series of pulses rather than being sharply attenuated. Finally, since the light pen is actually detecting light intensity pulses, the response time of the pen varies with the intensity of light from the CRT and the distance between the pen and the CRT and each pen must thereby have its sensitivity adjusted according to the intended usage for the pen.
In addition to the above described deficencies with prior art light pens, it should also be noted that there are a number of physical factors which affect the overall light pen design. Specifically, the ratio of the light intensity between a television screen at full brightness and at minimum usable brightness for a typical phosphor is greater than twenty thousand to one (20,000/1). This ratio is typically beyond the usable dynamic range of a silicon photo conductor. The CRT also radiates a strong electrostatic field which must be shielded from the sensitive photodetector circuit. This shielding thereby limits the minimum distance of the detector to the face of the CRT. In addition, the phosphor is on the inside of the glass screen and glass thickness varies with the type of display and with the position on the viewing surface. Therefore, the minimum distance from the end of the pen to the phosphor surface on the inside of the glass is not constant so that the field of vies varies as the tip of the pen is moved across the surface of the CRT.